1. Field of the Invention
The present invention relates in general to systems for interconnecting integrated circuits mounted on a circuit board and in particular to an interconnect system that provides a filter and impedance matching function.
2. Description of Related Art
Each node of an integrated circuit (IC) that is to communicate with external circuits is linked to a bond pad on the surface of the IC chip. In a packaged IC a bond wire typically connects the bond pad to a conductive leg extending from the package surrounding the IC chip. When the IC is mounted on a printed circuit board (PCB), the package leg is soldered to a microstrip PCB trace on the surface of the PCB. When bond pads of one or more other ICs mounted on the PCB are linked to the PCB trace, the bond pads, bond wires, package legs, and the PCB trace form an interconnect system for conveying signals between nodes of two or more ICs.
In high frequency applications the interconnect system can attenuate and distort signals. The conventional approach to reducing the amount of signal distortion and attenuation caused by the interconnect system has been to minimize the series inductance and shunt capacitance of the interconnect system. Much of the inductance comes from the bond wire and the package leg, and such inductance can be minimized by keeping the bond wire and package leg as short as possible. The capacitance of the bond pads can be reduced to some extent by minimizing the surface area of the bond pads. The capacitance of PCB traces can be reduced by appropriately choosing physical characteristics of the board including the size of the PCB traces, their spacing from ground planes and dielectric nature of the insulating material forming the circuit board. Vias, conductors passing vertically through a circuit board to interconnect PCB traces on various layers of the PCB, can be a source of capacitance at the PCB trace. Designers often avoid the use of vias in high frequency applications because vias can add substantial capacitance to the interconnect system. When vias are unavoidable, designers typically structure them so as minimize their capacitance. Minimizing the bond wire and package leg inductance and capacitances of the bond pad and PCB can help increase the bandwidth, flatten frequency response and reduce the signal distortion, but it is not possible to completely eliminate interconnect system inductance and capacitance. Thus some level of signal distortion and attenuation is inevitable when signal frequencies are sufficiently high.
What is needed is a way to substantially improve the frequency response of the interconnect system so as to reduce distortion and attenuation of high frequency signals.
The present invention is an improvement to a conventional system for interconnected integrated circuits (ICs) mounted on top of a printed circuit board (PCB). The conventional interconnect system employs a conductor (typically a bond wire and package leg) and a via to connect a bond pad on the surface of each IC to a PCB trace formed on the bottom side or an inner layer of the PCB so that the ICs can communicate with each other through the PCB trace. The inductance of the bond wires, package legs and vias, the capacitances of the bond pads, PCB trace and vias, the impedances of the IC devices connected to the pads, and impedance of the PCB trace cause the interconnect system to distort, reflect and attenuate signals as they pass between the IC bond pads.
In accordance with the invention, the interconnect system is improved by adapting it to provide a filter function that is optimized for the nature of the signals passing between the ICs by appropriately adjusting path inductances and capacitances relative to one another. The component values are adjusted to optimize relevant characteristics of the interconnect system frequency response. For example when the ICs communicate using low frequency analog signals where it is most important to avoid distortion, the xe2x80x9coptimalxe2x80x9d frequency response may have a narrow, but maximally flat, pass band. Or, as another example, when the ICs communicate via high frequency digital signals, the optimal frequency response may have a maximally wide passband.
It is accordingly an object of the invention to provide a system for interconnecting integrated circuits mounted on a printed circuit board wherein the frequency response of the interconnect system is optimized for the nature of signals passing between the ICs.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.